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Thermally responsive shape memory polymers (SMPs) used in 4D printing are often reported to be activated by external heat sources or embedded stiff heaters. However, such heating strategies impede the practical application of 4D printing due to the lack of precise control over heating or the limited ability to accommodate the stretching during shape programming. Herein, we propose a novel 4D printing paradigm by fabricating stretchable heating circuits with fractal motifs via electric-field-driven microscale 3D printing of conductive paste for seamless integration into 3D printed structures with SMP components. By regulating the fractal order and printing/processing parameters, the overall electrical resistance and areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentine structures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice, steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. We demonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching. Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D one with double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable the manipulation of the latter in a controlled and selective manner.

Microlenses (MLs) and microlens arrays (MLAs) have been fabricated by various manufacturing methods for use in many important micro-optical applications. It is still difficult for most fabricated MLs to meet design requirements, especially in displays, image-processing, illumination, and field of view (FOV) of imaging fields. Thus, flexible biconvex MLAs are preferred. Fabrication methods of flexible biconvex MLAs are complex, time-consuming, and costly. In this paper, we show that a method combining inkjet-printing and imprint-lithography is effective in fabricating flexible biconvex MLAs. Optimal flexible biconvex MLA fabrication conditions are determined by examining the pertinent parameters, including the preparation and testing of photoresist, the principle of the composite imprint-lithography process, and the alignment of biconvex MLAs during the imprint process. Finally, the main optical characterizations of flexible biconvex MLAs are tested, and the results indicate good optical quality. The results of the current study demonstrate that flexible biconvex MLAs can be used for the display and FOV of imaging fields.

In order to realize the mass production of the large-area flexible transparent film heater (FTFH) at low-cost, this paper presents a novel method which can achieve the direct fabrication of the large-area FTFH with Ag-grid by using an electric-field-driven jet deposition micro-scale 3D printing. The effects of the line width and the pitch of the printed Ag-grids on the optical transmittance and the sheet resistance are revealed. A typical FTFH with area of 80 mm × 60 mm, optical transmittance of 91.5% and sheet resistance of 4.7 Ω sq−1 is fabricated by the nano-silver paste with a high silver content (80 wt.%) and high viscosity (up to 20 000 mPa · s). Temperature-time response profiles and heating temperature distribution show that the heating performance of the FTFH has good thermal and mechanical properties. Furthermore, the adhesive force grade between the Ag-grid and the PET substrate measured to be 4B by 3M scotch tape. Therefore, the FTFH fabricated here is expected to be widely used in industry, such as window defroster of vehicles and display or touch screens owing to its striking characteristics of large area and low cost fabrication.

Flexible transparent electrodes (FTEs) with an embedded metal mesh are considered a promising alternative to traditional indium tin oxide (ITO) due to their excellent photoelectric performance, surface roughness, and mechanical and environmental stability. However, great challenges remain for achieving simple, cost-effective, and environmentally friendly manufacturing of high-performance FTEs with embedded metal mesh. Herein, a maskless, templateless, and plating-free fabrication technique is proposed for FTEs with embedded silver mesh by combining an electric-field-driven (EFD) microscale 3D printing technique and a newly developed hybrid hot-embossing process. The final fabricated FTE exhibits superior optoelectronic properties with a transmittance of 85.79%, a sheet resistance of 0.75 Ω sq-1 , a smooth surface of silver mesh (Ra ≈ 18.8 nm) without any polishing treatment, and remarkable mechanical stability and environmental adaptability with a negligible increase in sheet resistance under diverse cyclic tests and harsh working conditions (1000 bending cycles, 80 adhesion tests, 120 scratch tests, 100 min ultrasonic test, and 72 h chemical attack). The practical viability of this FTE is successfully demonstrated with a flexible transparent heater applied to deicing. The technique proposed offers a promising fabrication strategy with a cost-effective and environmentally friendly process for high-performance FTE.

Challenges remain regarding the defogging of the microlens array when it is applied to endoscopy, displays, camera lenses. A facile and convenient method of fabricating defogging microlens arrays directly on a substrate using electric field-driven jet printing is presented. By directly printing the microlenses and conducting silver wires onto the same substrate, we realized a microlens array (MLA) for defogging after electrification. The as-prepared defogging MLA exhibits homogeneous and stable heating performance. Furthermore, the practical applicability of defogging MLA is successfully demonstrated through an imaging and defogging test (MLA area: 18 * 9 mm, defogging time: 25 s, voltage: 3 V). This letter will provide a new and simple solution for the fabrication of micro-optical devices applied to foggy environments.

Nano-silver paste, as an important basic material for manufacturing thick film components, ultra-fine circuits, and transparent conductive films, has been widely used in various fields of electronics. Here, aiming at the shortcomings of the existing nano-silver paste in printing technology and the problem that the existing printing technology cannot achieve the printing of high viscosity, high solid content nano-silver paste, a nano-silver paste suitable for electric-field-driven (EFD) micro-scale 3D printing is developed. The result shows that there is no oxidation and settlement agglomeration of nano-silver paste with a storage time of over six months, which indicates that it has good dispersibility. We focus on the printing process parameters, sintering process, and electrical conductivity of nano-silver paste. The properties of the nano-silver paste were analyzed and the feasibility and practicability of the prepared nano-silver paste in EFD micro-scale 3D printing technology were verified. The experiment results indicate that the printed silver mesh which can act as transparent electrodes shows high conductivity (1.48 Ω/sq) and excellent transmittance (82.88%). The practical viability of the prepared nano-silver paste is successfully demonstrated with a deicing test. Additionally, the experimental results show that the prepared silver mesh has excellent heating properties, which can be used as transparent heaters.

Great challenges remain concerning the cost-effective manufacture of high-performance metal meshes for transparent glass heaters (TGHs). Here, a high-performance silver mesh fabrication technique is proposed for TGHs using electric-field-driven microscale 3D printing and a UV-assisted microtransfer process. The results show a more optimal trade-off in sheet resistance (Rs = 0.21 Ω sq-1 ) and transmittance (T = 93.9%) than for indium tin oxide (ITO) and ITO substitutes. The fabricated representative TGH also exhibits homogeneous and stable heating performance, remarkable environmental adaptability (constant Rs for 90 days), superior mechanical robustness (Rs increase of only 0.04 in harsh conditions-sonication at 100 °C), and strong adhesion force with a negligible increase in Rs (2-12%) after 100 peeling tests. The practical viability of this TGH is successfully demonstrated with a deicing test (ice cube: 21 cm3 , melting time: 78 s, voltage and glass thickness: 4 V, 5 mm). All of these advantages of the TGHs are attributed to the successful fabrication of silver meshes with high resolution and high aspect ratio on the glass substrate using the thick film silver paste. The proposed technique is a promising new tool for the inexpensive fabrication of high-performance TGHs.

Transparent conductive electrodes (TCEs) are widely used in applications ranging from display devices to thin-film solar cells. It remains a challenge, however, for industry and academia to implement mass production of large-area TCEs at low cost and at high volumes. It is also difficult to fabricate TCEs with good electrical properties and high light transmittance levels. In this study, a novel large-area and high-performance TCE manufacturing method is demonstrated that combines the electric-field-driven fusion direct printing technique and the liquid bridge transfer method. The master molds of the TCEs are fabricated by the electric-field-driven fusion direct printing technology, then soft lithography and the liquid bridge transfer processes are used to form the TCEs on different substrates. The relationships between process parameters, and their effects on the fabrication of the TCEs are explored through experiments, and the optimal process parameters are established. Using these methods, grid and stripe TCEs on the glass substrates with an area of 50 mm × 50 mm, an average line width of 4 µm, a spacing of 200 µm, a transmittance effectiveness of 88.94% and 88.15% which was nearly same as the bare glass, and a sheet resistance of 12 Ω sq−1 were successfully manufactured. These results illustrate the feasibility of mass producing large-area TCEs with high performances and at low cost.